1. Field of the Invention
The present invention relates to a magnetic sensor for sensing magnetic fields and a magnetic reading head for reproducing information recorded onto a magnetic disc, and particularly to a magnetic sensor and a magnetic reading head, which offer excellent reproduction and resolution.
2. Descriptions of the Related Arts
Magnetic recording/reproduction apparatuses have made remarkable improvements in their recording densities. Magnetic recording/reproduction heads thereof have been required to possess high performance in terms of both recording and reproduction characteristics. Concerning reproduction devices, improvements of three technical points have been demanded: (1) an improvement in a high sensitivity technique, (2) an improvement in a narrowing technique of a track width, and (3) an improvement in a narrowing technique of a reproduction gap. As to the point (1), the high sensitivity of the reproduction device has been advanced by improving a MR head utilizing a magnetoresistive effect. In a low recording density of several Gb/in2, the MR head converted a magnetic signal recorded on a recording medium to an electric signal by use of an anisotropic magnetoresistive effect (AMR), and when the recording density became high so as to exceed several Gb/in2, the MR head coped with this high recording density by use of a giant magnetoresistive effect (GMR) capable of producing higher sensitivity. With regard to the magnetic head utilizing the GMR, a structure called a spin valve is described, for example, in Japanese Patent Laid-Open No. 4(1992)-358310. The spin valve consists of a fixed layer made of a magnetic substance in which magnetization is fixed in a specified direction by an antiferromagnetic layer, and a free layer made of a magnetic film laminated on the fixed layer with a nonmagnetic thin film interposed therebetween. The spin-valve changes its electric resistance in response to a relative angle between the magnetization directions of the fixed and free layers.
However, with the recent development as to the high sensitivity of the reproduction device, a novel reproduction system for coping with further high sensitivity has been required. As likely reproduction systems, an advanced GMR effect called a specular-GMR or a NOL-GMR, in which a high spin polarization material or an oxide layer is sandwiched between interfaces of a GMR structure, and an increase of outputs is aimed at by a multi-reflective effect of spins of electrons; a GMR (CPP-GMR) which adopts a system for allowing a detection current to flow in a direction perpendicular to a film plane; and a tunneling magnetoresistive effect (magnetic tunnel junction effect) (TMR) are nowadays prospective.
These effects apply magnetoresistivity change phenomenon in a magnetoelectric effect. Herein, a Hall effect is also a kind of magnetoelectric effects, and a phenomenon in which when a magnetic field is applied in a direction perpendicular to a current flowing through a substance, a voltage is generated in a direction perpendicular to both of the current and the magnetic field. The Hall effect has been recognized for a long time, in which a carrier density and a scattering constant of electrons participate. The Hall effect has been applied to magnetic field measurements by use of a Hall generator formed of a semiconductor as that substance. The attempts to apply the Hall effect to the magnetic reading head are disclosed in Japanese Patent Laid-Open No. 2(1990)-308409 and the like. In this case, similarly to the Hall generator formed of a semiconductor, proposed are a system in which four electrode terminals as current terminals and voltage terminals are provided in a film plane and a signal magnetic field perpendicular to the film plane is measured; and a differential mechanism for sensing an inversion portion of a magnetic field by using a structure in which the two films are superposed thus forming an element having a thickness smaller than a bit interval of a magnetic recording. In Japanese Patent Laid-Open No. 9(1997)-289344, a fundamental principle for an in-plane magnetic recording is proposed as the magnetic field sensor utilizing an anomalous Hall effect of 3d-metal.
In a case where a structure of a reproduction device in future is considered, when the recording device adopts the conventional in-plane magnetic recording system, it is difficult for the recording device to produce a sufficient recording magnetic field, and the CPP-GMR and the TMR that are magnetic resistance sensors showing a high sensitivity are magnetic field sensors utilizing a structure for allowing a sensing current to flow perpendicularly to the film plane. Accordingly, it is anticipated that the reproduction device will switch over a structure of the CPP type through which a sensing current flows. However, when such a structure is adopted and a device area of the magnetoresistive sensor film is made small, a volume of a thickness of the magnetic film constituting the sensor film becomes small with the microfabrication and the thinness of the magnetoresistive sensor device, and its magnetization fluctuates by thermal fluctuation. Accordingly, a new problem that a predetermined magnetization ratio cannot be secured occurs.
When the Hall effect is utilized, materials of the current semiconductor Hall generator fundamentally show a linear output voltage for a magnetic field. Accordingly, the Hall generator demonstrates an advantage in measuring the magnetic field in a wide range. However, the Hall generator shows a small output for a micro magnetic field. Moreover, since these semiconductor materials are generally nonmagnetic (antimagnetic), it is difficult to concentrate a stray magnetic field from a recording medium on a sensing portion in the case where the magnetic field sensor is constituted by the Hall generator, and to enhance sensitivity. When the high-sensitive magnetic field sensor is fabricated with such a film, a structure capable of concentrating the magnetic filed on the sensing portion must be adopted, and a sensor film showing a large output for the magnetic field is necessary.
In consideration for the problems of the magnetic field sensor utilizing the Hall effect, an object of the present invention is to provide a magnetic field sensor having a structure which can be applied to magnetic reading heads and magnetic random access memories (MRAM).
To achieve the foregoing object, in the present invention, the magnetic field sensor is constituted by use of a material showing an anomalous Hall effect (magnetically induced Hall effect). This magnetic field sensor is applied with a magnetic field to detect a signal containing anomalous Hall effect components.
FIG. 1 shows a relation between a magnetic field B of a material showing the Hall effect and a material showing the anomalous Hall effect and a Hall voltage thereof. The conventional magnetic field sensor, particularly the Hall generator, which utilize the Hall effect (normal Hall effect), use a nonmagnetic semiconductor such as InSb. As apparent from FIG. 1, an output (Hall resistance) xcfx81H at this time is expressed as follows,
xcfx81H=RHB
where B is an applied magnetic field (magnetic flux density), and RH is a Hall constant. The output xcfx81H is a value in proportion to the applied magnetic field B, and the Hall constant RH is in inverse proportion to a carrier density of a substance forming the sensor. On the contrary, when a material showing the Hall effect is a magnetic substance, the output (Hall resistance) xcfx81H is expressed as follows,
xcfx81H=RHB+RSM
where B is an applied magnetic field (magnetic flux density), M is magnetization of the magnetic substance, and RS is an anomalous Hall constant. The output xcfx81H contains components in proportion to the magnetization M. Moreover, the anomalous Hall constant RS has a value anticipated by fluctuations of temperature and magnetization. Accordingly, in the magnetic substance having a soft magnetic property showing significant magnetic field dependency of the magnetization in a low magnetic field, since the output (Hall resistance) xcfx81H is in proportion to the magnetization M of the magnetic substance, a change of the anomalous Hall resistance in the low magnetic field, that is, a value of the anomalous Hall constant RS, is large. Accordingly, in the present invention aiming at the provision of the sensor film for sensing a low magnetic field, if a material containing a magnetic material is used as the sensor film and an anomalous Hall effect peculiar to the magnetic substance is utilized, there is a possibility that a higher output value can be obtained.
A relation among a current direction, a magnetic field direction and an output voltage direction has been heretofore obtained in such a manner that electrodes are provided at four spots in the in-plane, a magnetic field is applied in a film thickness direction while allowing a current to flow in the in-plane, and a voltage generated in a direction perpendicular to a direction of the current in the film plane is obtained as an output. In the case where application of a Hall generator using an ordinary film to a structure such as a magnetic head is considered, when a film plane is made to be in parallel with a medium plane, a structure in which an electrode is disposed closer to the medium than to a sensor film opposite to the medium must be adopted. Accordingly, a thickness of the electrode is superposed on a device floating height in the ordinary magnetic head and a distance (effective floating height) between the sensor film and the medium plane is made larger, so that a magnetic field reaching to the sensor film becomes weak. On the contrary, a structure was devised, which is capable of sensing the magnetic field from the medium with minimum loss by adopting a Hall generator structure without an electrode on the plane opposite to the medium. This is made possible by allowing a current or a voltage to flow or occur in the film thickness direction.
At this time, in the case of an anomalous Hall generator, since the sensor film alone is a magnetic film, this is a structure in which a magnetic field is apt to concentrate at the sensor portion. However, to enhance efficiency of the magnetic field concentration, a periphery thereof is constituted by a magnetic circuit structure, and a Yoke structure or a flux-guided type structure, in which a part thereof is constituted by the sensor film, is adopted. Accordingly, an output efficiency for the magnetic field can be improved.
As a material showing such an anomalous Hall effect, ferromagnetic metal by 3d-electrons, such as iron, cobalt and nickel, and alloys have been heretofore conceived. For example, since the anomalous Hall effect produced by a material added with impurities frequently tends to increase approximately in proportion to an impurity concentration and electrical resistance (there are a component in direct proportion to the impurity concentration and the electrical resistance and a component in square-proportion thereto), metallic materials added with impurity are conceived. As understood based on recent researches, in a part of materials such as LaSrMnO and LaCaMnO having a composition at a boundary area between an antiferromagnetic substance and a ferromagnetic substance, energy accompanied with an electron correlation rules physical properties of a system, and some materials show a critical state between an antiferromagnetic property and a ferromagnetic property in their certain composition. Thus, fluctuations of individual spins tend to be large, and at this time, a large anomalous Hall resistance may be observed. In a magnetic semiconductor constituted of a material obtained by combining a magnetic substance and a semiconductor, it has been found that a large moment and a large anomalous Hall effect can be observed. Particularly, in a diluted magnetic semiconductor obtained by doping a magnetic substance such as Mn into a III-V group compound semiconductor having a zincblende type crystal structure, a magnetic semiconductor obtained by doping transition metal into ZnO and GaN, and a magnetic semiconductor having the other zincblende type crystal structure, a high anomalous Hall effect material exists, which shows a ferromagnetic behavior at room temperature and has an anomalous Hall constant of 1xc3x9710xe2x88x9211 Vcm/AG (volt centimeter/ampere gauss) or more at room temperature. Accordingly, it is possible to fabricate a magnetic field sensor with a desired output or a magnetic device using this magnetic field sensor, by using these materials for the sensor portions.
The magnetic field sensor and the magnetic reading head of the present invention have the following features.
(1) A magnetic field sensor comprises: a film of a first electrode; a magnetic film showing an anomalous Hall effect, formed on the film of the first electrode; a film of a second electrode formed on the magnetic film; and third and fourth electrodes respectively connected to two regions provided so as to be apart from each other along a film plane direction of the magnetic film.
(2) In the magnetic field sensor defined in the item (1), the magnetic film is a ferromagnetic film or an antiferromagnetic film which contains at least one sort of elements selected among iron, cobalt, nickel and manganese.
(3) In the magnetic field sensor defined in the item (1), the magnetic film is a ferromagnetic film or an anti-magnetic film formed of a compound consisting of at least one sort of elements selected among iron, cobalt, nickel, manganese, vanadium and chromium, and a semiconductor material having a zincblende type structure, which contains at least one of gallium, arsenic, indium, antimony, silicon, germanium, tellurium, zinc oxide and titanium oxide.
(4) In the magnetic field sensor defined in the item (1), the magnetic film is a ferromagnetic film or an anti-magnetic film formed by laminating a semiconductor material which contains at least one sort of elements selected among iron, cobalt, nickel, manganese, vanadium and chromium, and which contains at least one of gallium, arsenic, indium, antimony, silicon, germanium, tellurium, zinc oxide and titanium oxide.
(5) In the magnetic field sensor defined in the item (1), the magnetic film is a ferromagnetic film or an antiferromagnetic film having a perovskite structure which contains elements not less than three, which are selected among lanthanum, strontium, calcium, manganese, boron, copper and oxygen.
(6) A magnetic field sensor comprises: a film of a first electrode; a magnetic film showing an anomalous Hall effect, formed on the film of the first electrode; a film of a second electrode formed on the magnetic film; and third and fourth electrodes respectively connected to two regions provided so as to be apart from each other along a film plane direction of the magnetic film, wherein a current is allowed to flow between the first and second electrodes and a voltage generated between the third and fourth electrodes is detected.
(7) A magnetic field sensor comprises: a film of a first electrode; a magnetic film showing an anomalous Hall effect, formed on the film of the first electrode; a film of a second electrode formed on the magnetic film; and third and fourth electrodes respectively connected to two regions provided so as to be apart from each other along a film plane direction of the magnetic film, wherein a current is allowed to flow between the third and fourth electrodes and a voltage generated between the first and second electrodes is detected.
The magnetic field sensor and the magnetic reading head should provide a magnetic domain-control layer for controlling Barkhausen noises, which is formed of a permanent magnet material and arranged on both sides of the magnetic film showing an anomalous Hall effect. The magnetic reading head of the present invention can be incorporated in a magnetic recording/reproduction apparatus which performs recording/reproduction for a magnetic disc.